Owing to its compact size, low weight and generally high efficiency, switching power supplies and converters have enjoyed ever increasing market adoption in the consumer electronics industry. This is especially so in portable applications where compact size, low cost, low weight and long battery life are all highly important considerations. Also, due to increased complexities and features of portable devices, there is an increased demand for different voltage levels within one portable device. For example, a display may require a different operating voltage than an interface product, which may require a different operating voltage than a micro-processor. The various operating voltages for each of building blocks of the portable device can be provided by different DC-DC voltage converters.
One well known example of the switching power supply and conversion is a simplified boost power converter schematic 1 shown in FIG. 1. An unregulated DC input 2, VIN, is boosted up into a regulated DC output 3, VOUT, through an energy storage and switching network having a Schottky diode 5, a vertical MOSFET 4 and a inductor 11. For those skilled in the art, the physical die of a vertical MOSFET has its source and drain built on the opposite surfaces of the die hence its device current flows in a direction perpendicular to the die surface, and this is why the device is referred to as a vertical device. A power regulating controller 13, whose internal details are only briefly illustrated, controls the gate 10 of the vertical MOSFET 4 with a grounded source 8 and a drain 9 connected to both the anode 6 of the Schottky diode 5 and the inductor 11. The regulated DC output 3, being connected to the cathode 7 of the Schottky diode 5, is also connected through a charging/discharging output capacitor 12 to ground 14. As illustrated, the vertical MOSFET 4 is a vertical N-channel FET with its device current flowing in a direction from the unregulated DC input 2 to ground 14. By way of an actual example, the unregulated DC input 2 can range from +2.7 to +5.5 volts from a Li-ion battery and the boosted up regulated DC output 3 can be adjusted up to +32 volts. For those skilled in the art, by simply replacing the vertical MOSFET 4 with a vertical P-channel FET and reversing the connection polarity of the Schottky diode 5, the boost power converter schematic 1 will then boost an unregulated negative input voltage (e.g., −3.3+/−10% volts) into a regulated negative output voltage of higher magnitude (e.g., −28 volts).
For embodiment into a physical product, the numerous integrated circuit (IC) dies corresponding to the Schottky diode 5, the vertical MOSFET 4 and the power regulating controller 13 need to be mounted onto different die pads to pack the semiconductor devices of the circuit into one package. At the packaging level, it is important to package the numerous IC dies efficiently with a reduced number of die pads and to use standard lead frames. The resulting benefits are reduced product size, lower cost, reduced time to market (by using standard available lead-frames) and in many cases include reduced circuit parasitics.
A prior art packaging example of FAIRCHILD FAN5606, a serial LED driver with current-regulated, step-up DC-DC converter, is illustrated in FIG. 2A. Notice that there are two die pads, die pad 20a and die pad 20b, located on a lead-frame 22. As a die-pad clearance 21 must exist between the die pads 20a and 20b, the resulting lead-frame 22 size has become larger than an otherwise lead-frame with a single die pad. Additionally although not directly visible here, minimum geometrical constraints of inter-die wire bonds could cause an even larger inter-die spacing hence further increases the resulting lead-frame 22 size and this will be presently illustrated.
A prior art multi-die package 35 of the boost power converter schematic 1 is illustrated in FIG. 2B. The Schottky diode 30 die typically comes with its cathode 30b as the bottom substrate and its anode 30a on the top. The vertical MOSFET 31 die typically comes with its MOSFET drain 31b as the bottom substrate and its MOSFET source and gate 31a on the top. Hence, two isolated die pads, Schottky diode die pad 30c and MOSFET die pad 31c, are required in the lead frame to seat the Schottky diode 30 die and the vertical MOSFET 31 die and to finally package them into one package covered with molding compound (not shown) before it is assembled onto a PC-board 33 with other circuit elements (such as inductors and capacitors) to complete the circuit. Additionally, bond wires 34a are required to effect an electrical connection between the anode 30a of the Schottky diode 30 and the MOSFET drain 31b of the vertical MOSFET 31 inside the package. As another illustrative part of the prior art multi-die package 35 although not essential for the understanding of the present invention, bond wires 34b are required to effect an electrical connection between the MOSFET source and a ground lead 32.
As a minimum clearance between the two isolated die pads (Schottky diode die pad 30c and MOSFET die pad 31c) must be maintained for isolation and product reliability, this results in a larger package size and the use of non-standard lead frame thus leads to higher cost. The required bond wires 34a further increases the cost of the package. Additionally, the bond wires 34a, owing to their associated parasitic capacitance and inductance, can also cause a degradation of the power conversion efficiency of the boost power converter. In essence, there exists a need to reduce complexity and space requirement of the prior art multi-die package 35.